.TH std::ranges::shuffle 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::shuffle \- std::ranges::shuffle

.SH Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::random_access_iterator I, std::sentinel_for<I> S, class
   Gen >
                                                                                (since
   requires std::permutable<I> &&                                           \fB(1)\fP C++20)
            std::uniform_random_bit_generator<std::remove_reference_t<Gen>>

       I shuffle( I first, S last, Gen&& gen );
   template< ranges::random_access_range R, class Gen >

   requires std::permutable<ranges::iterator_t<R>> &&                           (since
            std::uniform_random_bit_generator<std::remove_reference_t<Gen>> \fB(2)\fP C++20)
   ranges::borrowed_iterator_t<R>

       shuffle( R&& r, Gen&& gen );

   1) Reorders the elements in the given range [first, last) such that each possible
   permutation of those elements has equal probability of appearance.
   2) Same as \fB(1)\fP, but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   first, last - the range of elements to shuffle randomly
   r           - the range of elements to shuffle randomly
   gen         - the random number generator

.SH Return value

   An iterator equal to last.

.SH Complexity

   Exactly (last - first) - 1 swaps.

.SH Possible implementation

   struct shuffle_fn
   {
       template<std::random_access_iterator I, std::sentinel_for<I> S, class Gen>
       requires std::permutable<I> &&
                std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
       I operator()(I first, S last, Gen&& gen) const
       {
           using diff_t = std::iter_difference_t<I>;
           using distr_t = std::uniform_int_distribution<diff_t>;
           using param_t = typename distr_t::param_type;
           distr_t D;
           const auto n {last - first};
           for (diff_t i {1}; i < n; ++i)
               ranges::iter_swap(first + i, first + D(gen, param_t(0, i)));
           return ranges::next(first, last);
       }

       template<ranges::random_access_range R, class Gen>
       requires std::permutable<ranges::iterator_t<R>> &&
                std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
       ranges::borrowed_iterator_t<R> operator()(R&& r, Gen&& gen) const
       {
           return (*this)(ranges::begin(r), ranges::end(r), std::forward<Gen>(gen));
       }
   };

   inline constexpr shuffle_fn shuffle {};

.SH Example


// Run this code

 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <random>

 void print(const auto& a)
 {
     for (const auto e : a)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

 int main()
 {
     std::array a {'A', 'B', 'C', 'D', 'E', 'F'};
     print(a);

     std::random_device rd;
     std::mt19937 gen {rd()};

     for (int i {}; i != 3; ++i)
     {
         std::ranges::shuffle(a, gen);
         print(a);
     }
 }

.SH Possible output:

 A B C D E F
 F E A C D B
 E C B F A D
 B A E C F D

.SH See also

   ranges::next_permutation generates the next greater lexicographic permutation of a
   (C++20)                  range of elements
                            (niebloid)
   ranges::prev_permutation generates the next smaller lexicographic permutation of a
   (C++20)                  range of elements
                            (niebloid)
   random_shuffle
   shuffle                  randomly re-orders elements in a range
   \fI(until C++17)\fP            \fI(function template)\fP
   \fI(C++11)\fP
